This invention relates to blow molded containers and, specifically, to blow molded ultra thin film containers, such as non-self-supporting plastic bags which have a thickness in the range of about 1 to 10 mil. The invention also relates to a method and apparatus for forming such containers.
In the past, it has been the conventional practice to manufacture non-self-supporting thin film bags by a continuous blown film process where an extruded tubular parison is blown radially outwardly to a desired thin wall configuration without the use of molds or dies. The tubular parison is subsequently flattened, or collapsed, and perforations or score lines and heat sealed bottom seams are formed at spaced locations to define individual bags. The continuously formed bags are then rolled up on a take-up roll or drum.
It has also been attempted to blow mold thin film containers. In this process, a parison is blown outwardly into conformance with container-shaping surfaces provided within the interior of a blow mold cavity. However, it has not been possible in the past to form a high quality, effective heat seal on the top, around or across the bottom of the bag in light of the relatively cooled condition of the blow mold halves. In other words, even if the parison remains in a heated condition, contact by the parison with the otherwise relatively cool blow mold halves, draws sufficient heat away from the bottom seam area of the bag to prevent an effective heat seal from being formed.
The present invention provides apparatus and a related process for blow molding an ultra thin plastic film bag wherein the mold apparatus is uniquely configured to generate sufficient heat in the process to form an effective heat seal or weld along the edge of the bag without increasing the wall thickness of the bag in the seam area.
In one preferred embodiment of the invention, the blow mold halves are provided with laterally aligned, outwardly extending projections intermediate upper and lower edges of vertical wall surfaces defining the parting line of the mold halves. These projections serve to pinch off the parison during closure of the mold halves. The projections also form, in conjunction with vertical surfaces of the mold halves, upper and lower cavities above and below the laterally aligned projections. The upper cavity results in a hollow groove being formed along the length of the pinch off projections during blow molding. The lower cavity is no wider than the upper cavity and should have a width less than twice the parison wall thickness so that upon closure of the mold halves, the parison flash material below the pinch off projections is compressed out of the lower cavity. It is this squeezing or compression of material which generates heat sufficient to raise the temperature of the plastic material in the upper cavity above its melting point. As a result, an effective heat seal or weld is formed along the mold parting line, just above the pinch off projections.
It will be understood that the width and depth of the upper cavity and the width of the lower cavity will be determined by the particular plastic material used in the process.
The pinch off projections are identically formed, each having a horizontally projecting surface, and a downwardly and inwardly inclined surface, the intersection of the two forming a pinch off edge. The angle which the inclined surface makes relative to the horizontal surface is also important to the integrity of the heat sealed seam, as will be explained in greater detail further herein.
Thus, in one aspect of the present invention a method of forming an ultra thin film plastic container and heat sealing the seam thereof comprising the steps of:
a) forming a plastic parison of a first predetermined thickness;
b) providing a pair of blow mold halves, each of which is formed with a vertical parting line surface including a pinch-off projection intermediate upper and lower edges of the parting line surface;
c) closing the blow mold halves about the parison;
d) blow molding the parison within a mold cavity formed by the blow mold halves to form an ultra thin film plastic container having a second predetermined thickness less than the first predetermined thickness;
wherein step (b) is carried out by shaping the pinch-off projections and upper and lower portions of the parting line surfaces to generate sufficient heat during steps (c) and (d) to cause the parison to reach a temperature at least equal to a melting point temperature of the plastic to thereby provide an effective heat seal along a seam immediately adjacent the pinch-off projections.
In another aspect, the present invention provides a blow mold construction comprising a pair of mold halves which together form a mold cavity, each of which includes a parting line surface around the mold cavity, the parting line surface at one end of the mold having an upper portion, a lower portion, and an intermediate projecting portion, the pair of mold halves together forming an upper recess in the mold cavity and a lower recess external of the mold cavity, wherein the width of the lower recess is selected so that it is less than two times the wall thickness of a parison to be blow molded within the cavity.
In still another aspect, the present invention provides an ultra thin film plastic container comprising a one piece tubular member having a wall thickness of from about 1 to about 10 mil, and a seam along at least a portion of a bottom edge, said seam located within a hollow groove.
Additional objects and advantages of the present invention will become apparent from the detailed description which follows.